A variety of bis compounds are known to be useful as sensitizers for photopolymerization of ethylenically unsaturated compounds. Benzophenone, with or without substituents, is one that is widely used. Others include ##STR2## wherein R.sup.11 is a halogenated alkyl, ALK is preferably a divalent alkyl, as shown, for example, in U.S. Pat. No. 4,040,923, issued on Aug. 9, 1977; bis(substituted diphenyl) alkyleneamines such as are disclosed in U.S. Pat. No. 4,035,189, issued on July 12, 1977; bis-aryldicarbothioic acid esters as disclosed, for example, in U.S. Pat. No. 4,012,302, issued on Mar. 15, 1977; azo-bis and hexa-aryl bis compounds such as are disclosed in British patent specification No. 1,504,616; and bis compounds containing non-adjacent keto groups such as are disclosed in U.S. Pat. No. 3,628,962, issued on Dec. 21, 1971.
However, each of the aforementioned is disclosed for use only with specific compounds, such as epoxy compounds or those having ethylenic unsaturation, or with polymers such as nylon which have had to be modified to include an appended sensitizable adjunct. Thus, polymers have been rendered photocrosslinkable, when used with a sensitizer, by adding to the polymer a specific crosslinkable pendant moiety, such as one containing ethylenic unsaturation. Without such expensive modification of non-light-sensitive compounds, such latter compounds are not useful in a photocrosslinkable system.
What has long been desired is a photoinitiator or sensitizer which will permit a large variety of compounds, including those not ordinarily light sensitive, to be photopolymerized or photocrosslinked. The result would be a greater variety of compositions useful in preparing an inexpensive litho printing plate, photoresist, or light-hardenable coating. No longer would it be necessary to rely, as in the past, on expensive, tailor-made photosensitive polymers.
Yet another difficulty common with some sensitizers is that the sensitizer transfers its photoactivated energy via a triplet-triplet energy exchange, leaving the sensitizer with its original chemical structure intact after the exchange. The continued absorption of the sensitizer at that portion of the composition makes it difficult for portions underneath to be exposed, e.g., as would occur for thicker layers. The result is an effective limitation on the thicknesses that can be coated and effectively exposed.